The K9AY loop antenna for 160 and 80m bands

You don't have the space to put up a 4 beverage antenna's, an EWE or a 4-square and want 'good ears' on 160 and 80m? No problems ... try a K9AY loop antenna !

The K9AY only needs about 10m of space... compare vs. 4 EWE antennae!

The V-radiation pattern. More gain over good ground (solid graph), but F/B ratio is best over lossy ground

This is a very performing receiving antenna for frequencies up to 4 Mhz, which exhibits an impressive F/B ratio, especially on MF. While using the antenna, you will note that the 3 positions do not provide much signal difference (very broad receiving range) but the 4th position will always reveal it's excellent F/B characteristic claimed up to 30 dB. As with all small loop antenna's, it is recommended to boost the signal by inserting a preamplifier with a gain of at least 15 dB. I found a good schematic of preamplifier, offering up to 20 dB gain, a high 3rd order intercept point, and a bandpass filter at the input rejecting signals from the AM broadcast band and above 4MHz. See details here.

It is a as well an excellent antenna (some OM's say 'miraculous') for MW broadcast DX'ing.  Listen to the following examples (mp3 files) where you will clearly hear 2 different stations, on the same frequency, while I am playing with the receiving direction   example 1   example 2

As there is much literature on the net about this antenna,  I will not publish it here, but just provide some pictures on my construction. Download the original paper (pdf document) from K9AY here.

Construction of K9AY antenna:

The antenna is designed as a portable antenna, for setup during winter months.

The temporary setup in the garden to test / tune the construction.

The antenna pole is a SPIDERBEAM fibreglass  mast of 12m where only the first 8m are used.

The wires are 2x 26m loops of 2.5mm² PVC insulated flexible copperwire.

Detail of the top construction. It is a 'hat' of PVC tube fitting over the top section, on which an insulator is mounted.
The switchbox is designed to be mounted on any mast base and held in place with tie-straps. The 2 relays are 1.5 kV 7.5A vacuum models out of a junk box, they are switched by AC voltage fed on the coax line.  All hardware is stainless steel.

For the initial tests, the termination resistor is fixed at 470 Ohm (2 watt carbon). For optimal performance, I guess this must be made variable by a 8-step fixed resistor combination (from 340 to 680 ohms) or a 'vactrol' device (LDR with LED combination)

The control box front panel. I added a possibility to switch over from the K9AY to a TX antenna and as well switch on a preamp.

Particular care was taken to provide RF-clean switching and power signals. I however noted a small hum component when the antenna is switched in NW position (in this case, full AC is on the antenna line, through the 0,1 µF caps)

The control box rear panel. I have provided  key IN / OUT terminals for RX/TX control (PA) , preamp power, and SO-239 for the transceiver, TX antenna and K9AY.

When the control box is not powered, in 'through' mode, or in TX state, the transceiver is linked to the TX-ant.  This is being taken care by another 1.5 kV 7.5A vacuum relay.


Optimal termination resistance of K9AY antenna:

As the first results with my K9AY showed an excellent F/B on MW and 160m, but much less pronounced on 80m, I suspected that my termination resistor (the recommended 'average 470 Ohm') was not optimal.

I therefore made a VACTROL and spent a couple of hours trying to measure the optimal value in function of frequency....

These measurements are indeed not easy !

  • The value of the resistance has an effect on the loading of antenna, thus signal strength, irrespective of F/B.
  • The F/B variation is not clearly discernable, unless signals are really from the exactly opposite direction, and coming in on the right angle
  • Strong fading on these bands

Anyhow, I have managed to measure the optimal R at four frequencies, see the graph, which is consistent and confirms the values recommended (160m=390 Ohm, 80m=560 Ohm).

Now, is it really worth working with a VACTROL or remote switchable resistor bank ? In my opinion, no seems a little overkill:
  • The F/B performance is excellent on MW and on 160m, on 80M this less pronounced. The variation of termination R on 160m has some effect, but it is not dramatic. On 80m it has only a very very limited effect! Above 4MHz, it seems to have no more discernable effect...
  • So consider the K9AY as an excellent  antenna for 160m, where you can boost reception by a couple of dB in the 'front' direction and really reject QRM up to >30 dB in the 'back'. Even with R=560 Ohm (=value for 80m) , the F/B is still 25 dB or so ....(according to what I hear and see on my S-meter). Under favorable conditions (angle, direction), I have seen QRM signals of S9 vanish to S2, way down in my background noise .... I must say very impressing!
  • On 80m, with the optimal termination, I estimate the F/B to be 15 to 20 dB maximum. Possibly my ground was not optimal, so I tried with 4 ground wires of 7m long, laid under the loops, and connected to the relay switch box ground. In fact this had very little effect on the F/B on 80m, which remains well under the 30 dB or more on 160m.

As the remote switching solution adds complexity / fragility and requires additional cables I would rather work with a miniature switch on the control box, by which the choice 160 / 80m is made.

I think that - for my activities - 90% of the time it would be in the '80m' position, even for 160m, unless working in conditions where the optimum F/B would be really required (contest ?)

Off course, this can be different for you .... I imagine that if you are interested in MW, this termination resistor fine-tuning  could  make the difference between reception of 2 stations on same QRG! But then you should as well consider installing 2 systems to cover all directions with the best possible F/B....

Overall impression : When switching over from the K9AY to my vertical HF6V with 100 radials on 80m, the reception is certainly quiter (improved S/N), but I cannot say that I could copy  every day sigs on the K9AY which were impossible to decode on the vertical. The K9AY's main advantage is more  it's ability to reject signals from unwanted directions. All by all, it is a performing antenna because of low-cost, portable, and very compact.